Land use effects on the dynamics of dissolved organic matter and biological nutrient recycling in south-central Ontario streams

It is well recognized that human alteration of the terrestrial landscape can strongly impact the function of stream ecosystems. Although these activities are known to strongly impact rates of nutrient loading (N and P) to streams, the impact of anthropogenic land uses on the transport of carbon (C) has not been identified. In addition, despite our understanding that human land uses strongly impact nutrient loading, it remains poorly understood how in-stream biological nutrient dynamics are impacted. This thesis addresses these knowledge gaps.;The results of this thesis indicate that a variety of stream ecosystem functions are impacted by agricultural land use activity. This research indicates that not only are nitrogen concentrations increased in watersheds with more agricultural land use, but also that carbon dynamics and nutrient recycling processes are altered.;Dissolved organic matter (DOM) dynamics were examined over multiple years in streams selected across a land use gradient in south-central Ontario. Finer spatial scale, longitudinal DOM dynamics were examined within a mixed land use watershed. Finer temporal scale DOM dynamics were examined over a diel period in a subset (6) of streams with varying land use. Spatial differences in DOM concentration (measured in units of carbon; DOC) did relate to watershed landscape character, but agricultural land uses did not appear to strongly influence concentration. In contrast to concentration, spatial differences in DOM character appeared to be influenced strongly by the amount of continuous cropland within watersheds. Fewer wetlands and increased nitrogen loading in these watersheds were associated with the shift in DOM character. As indicated by the relationship between the fluorescence index and the proportion of each stream's riparian area under continuous cropland (r 2 = 0.81), DOM in more intensively agricultural watersheds was more microbially derived. DOM character also varied temporally, with more adsorptive DOM being exported during periods of higher soil moisture and after storm events. Agricultural land use was observed to strongly impact how nutrients are recycled through stream fish assemblages in the region. For both ammonia (NH4+-N) and phosphate (SRP) nutrients, total assemblage excretion exhibited a strong positive relationship with the amount of cropland use in each watershed. The distance required for fish assemblages to turnover ambient nutrient pools was shorter and the ratio of excretion rate to nutrient demand was higher for cropland-dominated streams. These patterns appeared to be driven largely by a shift in fish assemblage structure to lower trophic levels with increasing stream productivity.